Air supply systems on board a passenger aircraft have in recent years become increasingly more complex, as different climate zones in the aircraft cabin must be supplied with fresh air in order to make the flight as pleasant as possible for the passengers. The cabin air must also be treated at continuous intervals. For this purpose the used cabin air is removed from the cabin, mixed with fresh air and supplied to the cabin again. Furthermore, certain components which are installed on board, such as, e.g. the air conditioning system, must be supplied with cooling air. The installation space of the air conditioning system must be ventilated in order to remove any escaping fuel and/or oil vapours and to exclude potential fires.
For this reason concepts which provide solutions for the fresh air supply, circulating air circulation, cooling air supply and ventilation of the installation space of the air conditioning system have been developed in recent years. These solutions are described below with reference to FIG. 1 to FIG. 5. In this respect only the principles which form the basis of the individual solutions are described, without entering into technical detail.
A) Fresh Air System
FIG. 1 shows a conventional solution 10a for the fresh air ventilation of the aircraft cabin. For this purpose the fresh air is obtained during flight as bleed air from the primary circuit 12 (high- and intermediate-pressure compressor) of the main turbine engines and on the ground from the auxiliary turbine 18. On account of the high temperature level, the bleed air from the primary circuit 12 of the main turbine engines must be precooled. This precooling takes place through bleed air from the secondary circuit 14 of the main turbine engines which flows through a heat exchanger 16 and as a result pre-cools the bleed air obtained from the primary circuit 12 of the main turbine engines. The air conditioning system 20 of the aircraft is in addition driven by the flow energy of the bleed air from the primary circuit 12 of the main turbine engines.
In the conventional fresh air system which is shown in FIG. 1 the withdrawal of bleed air from the primary circuit of the main turbine engines leads to a power loss and to increased kerosene consumption by the main turbine engines. Furthermore, an increased construction expenditure is required on account of the high temperature and pressure level at the bleed air off-takes and temperature monitoring in order to protect surrounding structures. It is also necessary to control the volumetric flow rate and the pressure of the withdrawn bleed air through appropriate valves. This leads to further possibilities for failure of the electromagnetically controlled valves, which must be compensated by redundancies, which in turn increases the construction expenditure. Moreover, the fresh air may be contaminated through the escape of oil at the turbine engines and the auxiliary turbine.
A more recent concept 10b for the fresh air supply which is provided for future aircraft projects is represented in FIG. 2. According to this concept 10b, which is not yet in use, fresh air is supplied from outside of the fuselage through a separate ram air inlet opening 22. The ram air entering the ram air inlet opening 22 is compressed by means of an electrically driven compressor 24 in order thus to provide the flow energy for operating the air conditioning system 20. The drive energy for the compressor is provided during flight by the generators of the main turbine engines and on the ground by the auxiliary turbine. This concept does not include the withdrawal of bleed air from the primary circuit of the main turbine engines and the auxiliary turbine.
The fresh air supply without bleed air which is represented in FIG. 2 requires the provision of a homogeneous inflow, which means that a complex intake duct geometry at the ram air inlet 22 is necessary. In addition, the ram air inlet 22 increases the drag coefficient of the aircraft. The complexity and therefore the susceptibility to failure of the system are increased by the controlled actuators for opening and/or closing the inlet flaps at the ram air inlet 22. When the aircraft is de-iced on the ground there is also the risk of de-icing fluid passing from the fuselage into the ram air inlet duct 22 and the fresh air being contaminated as a result.
B) Circulating Air System
With regard to recirculation of the circulating air and its feed into the fresh air system of the aircraft, according to current system concepts 30, as represented in FIG. 3, the re-circulated circulating air 36 is fed into the fresh air system downstream of the air conditioning system 20. The feed and mixing of the re-circulated circulating air 36 with the fresh air delivered by the air conditioning system takes place in a mixing chamber 32. The fresh air mixed with the circulating air is routed from the mixing chamber 32 into the aircraft cabin. The flow energy for the circulating air ventilation is provided by electric blowers 34.
In terms of its construction as a rotating mechanical element, the circulating air blower 34 shown in FIG. 3 for re-circulating the circulating air 36 represents a potential source of failure, which has lasting effects on the fail safety of the system and must be compensated through sufficient redundancies with a corresponding weight disadvantage. Moreover, a mixing chamber 32 is necessary in order to mix the fresh air which is delivered by the air conditioning system 20 with the re-circulated circulating air 36.
C) Ventilation/Cooling Air System
As already mentioned above, the installation space of the air conditioning system must be ventilated in order to remove any escaping fuel and/or oil vapours and to exclude potential fires. A conventional system solution 40a, as represented in FIG. 4, provides a ventilation system in which the ventilation air is supplied from outside of the fuselage through a ram air inlet 42 in the vicinity of the air conditioning system. The ventilation of the installation space 44 of the air conditioning system during ground operation of the aircraft is secured by a compressor 48. The compressor 48 obtains its drive energy from the high-pressure bleed air system 52 which drives a turbine 50. Exhaust air is channelled via an outlet opening 46. The compressor 48 must not be operated during flight on account of the damming effect of the cooling air at the ram air inlet 42.
The ventilation system which is represented in FIG. 4 requires a ram air inlet 42 which is provided separately from the other systems and which in turn has a lasting influence on the drag coefficient of the aircraft. The compressor 48, which is necessary in this system, for ground operation, when no ram air is available, with air instead being sucked in from outside by means of the compressor 48 through the ram air inlet 42, represents a possible source of failure in addition to the actuators and the ram air inlet flaps.
Another concept 40b with regard to the cooling air supply for the air conditioning system and the ventilation of the installation space of the air conditioning system is represented in FIG. 5. According to this concept 40b, the ventilation air for the installation space of the air conditioning system 44 is withdrawn from the ram air inlet 42 of the cooling air blower 54. The ventilation air is removed on account of the flow conditions at the boundary layer of the fuselage, i.e. through negative pressure in the wake region of the cooling air outlet 46.
In the system which is represented in FIG. 5 the outlet geometry in the wake of the cooling air outlet 46 must be of a complex design in order to ensure that the ventilation air is sucked off. The flow conditions in the wake of the cooling air outlet, i.e. in the boundary layer of the fuselage, are neither predictable nor controllable. This circumstance therefore represents a further unsafety factor which leads to safety hazards in view of the possibility of fuel escaping or oil evaporating in the installation space of the air conditioning system.
DE 2 907 826 A describes an air circulating-cooling system for an aircraft in which, by means of an ejector mixing stage, fresh air withdrawn as bleed air is mixed with circulating air re-circulated from the passenger cabin. Mixing of the fresh air with the circulating air occurs downstream of a heat exchanger which is cooled through the use of ram air.
The air conditioning system described in DE 103 01 465 A1 includes at least two air conditioners, each of which is connected at its inlet with a supply line and its outlet with a cabin to be air-conditioned or a mixing chamber. The fresh air is withdrawn as bleed air from the turbine engines. Each of the air conditioners is driven through ram air for cooling the bleed air.
EP 1 695 910 A2 describes a system for generating inert gas. The system, for example, creates nitrogen gas for the cargo space or the fuel tank of the aircraft. To this end, compressed cabin air is pre-cooled, subsequently compressed again and directed to a main heat exchanger, before it reaches an air-disaggregation module which splits the cooled cabin air into inert gas and a permeate. The permeate is subsequently supplied to a ram air duct and discharged into the atmosphere. In order to increase the flow rate through the heat exchanger, ejectors are utilised which create at the outlet end of the heat exchanger a low pressure region.
U.S. Pat. No. 2,491,461 discloses a pressure generating system for an aircraft, in which, inter alia, circulated cabin air is mixed with compressed fresh air upstream of an air conditioner.
The object of the present invention is therefore to provide an air supply system for an aircraft, in particular for a passenger aircraft, with increased fail safety, in which the number of fail-unsafe components, such as, e.g. blowers and compressors, is minimised.